The invention is based on the priority application EP 06290827.2 which is hereby incorporated by reference.
The present invention relates generally to optical transmission systems and more particularly to a method and system for optical transmission signal level configuration.
Radio frequency RF communications systems, e.g. in building and in tunnels, which are distributed over large distances, typically use an optical fibre transport network due to the great advantage of the low loss characteristics of the optical fibers.
FIG. 1 shows a block diagram of a typical RF communications system with analogue optical transport network in downlink DL and uplink UL direction. In downlink DL an RF signal RFi is provided to a transmitter device TX1 in charge of adapting the signal to be transmitted in optical form, over an optical fibre link OF1, to a receiver device RX1, which receives the optical signal and provides the corresponding RF output signal RFo. The optical transport network transmitter device TX1 comprises a variable attenuator AT1 to limit the signal level at the input of an optical transmitter OTx1 which typically comprises a laser and sometimes also a pre-amplifier. On the other side of the optical fiber, the receiver device RX1 comprises an optical receiver ORx1 followed by another variable attenuator AT2 which provides the desired signal level to a power amplifier PA1. In uplink direction UL, an RF signal RFi′ is provided to a transmitter device TX2 in charge of adapting the signal to be transmitted in optical form, over an optical fibre link OF2, to a receiver device RX2, which receives the optical signal and provides the corresponding RF output signal RFo′. The optical transmitter device TX2 comprises a power amplifier PA2 followed by a variable attenuator AT3 which provides a fixed signal level at the input of the optical transmitter OTx2. On the other side of the optical fiber, the receiver device RX2 comprises an optical receiver ORx2 followed by another variable attenuator AT4 which provides the desired signal level to a determined input 3 of a signal combiner RC. The signal combiner RC comprises more inputs 1, 2, 4 intended for other uplink optical links (not shown) parallel to the one shown in the example of the figure.
The variable attenuators AT1 to AT3 are placed in the optical network in order to avoid signal distortion from over-drive of the optical transmitters OTx1, OTx2 and power amplifier PA1 by too large signal levels. In the uplink UL, the attenuator AT4 provides a fixed signal level to a certain input 3 of the combiner RC so that at the output of the combiner RC a predetermined fixed signal level (the sum of the different uplink optical signals) is achieved and all output signals of the parallel optical links can be set to the same level.
A problem with the known analogue optical system described above is that for network commissioning, i.e. to put the optical network ready for service, or for maintenance or failure network test, the optical network attenuators AT1 to AT4 shall be, each time, set to the optimum level so that the signal at their output has a determined value. This is done, for example, in downlink DL as well as in uplink UL, by providing a source S1, S2 signal to the optical network, reading the signal level at the output of the attenuator, the power amplifier PA1 and/or the combiner RC e.g. by means of a signal monitoring equipment AN1 to AN4, and setting the attenuators to the needed level. To perform the steps above, a number of human personnel is needed to provide a signal source, to go to the place the attenuators AT1 and AT4 are located, connect and read the monitoring equipment AN1 to AN4 and manually leveling the attenuators. If we take in consideration that such optical network systems are distributed over large areas with long distances between locations of signal input and output, it is clear that a lot of human effort is needed and some practical problems may arise when the system is installed in a number of tunnels or in great buildings e.g. locating the attenuators, and communication between personnel. An alternative to avoid much human personnel would be to provide in a single place with display information about the source signal S1 and S2, the signal at the output of the attenuators AT1 to AT4 and means to remotely actuate on the attenuators to set them to the optimum level. The latter could be done with a single person but would increase the complexity and cost of the optical network and in any case, variations of the optical network system and system parameters e.g. such as the number of RF carriers needs interaction and adjustment.